Both AC and DC generators may be controlled by regulating the current flowing through a generator field winding. This current is generally supplied though a controlled transistor, frequently a MOSFET. By controlling the field transistor, the generator field current is modulated based on demand to maintain the generator output voltage at a desired level.
One failure mode for a generator may be referred to as an “out of control” failure mode. In this failure mode, a generator control unit fails to control the generator excitation current, and the generator output voltage quickly reaches dangerously high levels. This may damage equipment connected to the generator. The cause of this failure mode may be, for example, a shorted field transistor, or a short of any components that are connected in parallel with the field transistor, or a connector pin short. Because the field switch is a semiconductor device, this is not an uncommon failure mode.
When a short circuit occurs, the field is not controlled and becomes fully excited. As a result, the output voltage of the generator rises quickly to a very high level until the generator becomes saturated. This failure mode can cause problems in constant frequency electrical systems. However, the severity of this failure mode is even greater in variable frequency electrical systems such as those sometimes found on aircraft. Such variable frequency electrical systems are becoming increasingly popular because of their overall lighter weight and increased efficiency.
In a variable frequency system, the generator may operate at frequencies nearly twice as high as the frequencies used in constant frequency systems. The higher the frequency at which a generator operates, the shorter the time it will take to reach an overvoltage condition. Therefore, effective protection against this failure mode in variable frequency systems is becoming an important concern.
Some systems now require generators that limit overvoltage to about 150 V rms for 115V AC electrical systems and to 300V rms for 230V AC electrical systems. A conventional approach to overvoltage protection is to monitor voltage levels and disconnect the field winding when an overvoltage is detected. This approach, however, is too slow to provide effective protection for the above failure mode, especially in a variable frequency system.
FIGS. 4a and 4b illustrate a simulated response to an overvoltage condition by a conventional voltage protection circuit used with a generator operating at 115V, 700 Hz with a light load of 7.5 kW. FIG. 4a is a graph of generator field current having a nominal level of 0.4 amps. FIG. 4b illustrates the average voltage level of the three-phase power supply. A fault occurs at time t1 which leads to an increase in the current level to about 3.2 amps at time t2. At time t3, the generator field voltage peaks at 248.5 V or 176 V rms, well above the 150V rms limit often required in applications employing variable frequency generators. It is therefore desirable to provide a method and apparatus for addressing this failure mode in a manner that limits generator overvoltage and protects components connected to the generator.